Co-co assembly for oligomers Most of the human proteome forms oligomeric protein complexes, but how they assemble is poorly understood. Bertolini et al. used a ribosome-profiling approach to explore the existence of a cotranslational assembly mode based on the interaction of two nascent polypeptides, which they call the “co-co” assembly. Proteome-wide data were used to show whether, when, and how efficiently nascent complex subunits interact. The findings also show that human cells use co-co assembly to produce hundreds of different homo-oligomers. Co-co assembly involving ribosomes translating one messenger RNA may resolve the longstanding question of how cells prevent unwanted interactions between different protein isoforms to efficiently produce functional homo-oligomers. Science , this issue p. 57